Among the Greeks, harmony and the soul were considered to be similar in nature, because all souls are harmonic. The hexad is also the symbol of marriage, because it is formed by the union of two triangles, one masculine and the other feminine. Among the keywords given to the hexad are: time, for it is the measure of duration; panacea, because health is equilibrium, and the hexad is a balance number; the world, because the world, like the hexad, is often seen to consist of contraries by harmony; omnisufficient, because its parts are sufficient for totality (3 +2 + 1 = 6); unwearied, because it contains the elements of immortality.
By the Pythagoreans the heptad--7--was called "worthy of veneration." It was held to be the number of religion, because man is controlled by seven celestial spirits to whom it is proper for him to make offerings. It was called the number of life, because it was believed that human creatures born in the seventh month of embryonic life usually lived, but those born in the eighth month often died. One author called it the Motherless Virgin, Minerva, because it was nor born of a mother but out of the crown, or the head of the Father, the monad. Keywords of the heptad are fortune, occasion, custody, control, government, judgment, dreams, voices, sounds, and that which leads all things to their end. Deities whose attributes were expressed by the heptad were ŧis, Osiris, Mars, and Cleo (one of the Muses).
Among many ancient nations the heptad is a sacred number. The Elohim of the Jews were supposedly seven in number. They were the Spirits of the Dawn, more commonly known as the Archangels controlling the planets(s).
Source: http://www.sacred-texts.com/eso/sta/sta16.htm